摘要
当前，在解决全球能源危机与实现社会可持续发展的双重难题下，清洁、环保、高效的新能源的利用面临机遇与挑战。太阳能作为一种取之不尽的清洁能源受到各国重视。利用太阳能最直接有效的方式是光伏转换，即将太阳能直接转换为电能。在每个太阳电池器件中，光伏玻璃作为一种具有高透光率的玻璃基体对提高光伏电池的光电转换效率具有重要作用。传统的光伏玻璃表面至少会有8%的入射光被反射而无法利用(70度角入射时，反射率增至27%)。因此设计出具有减反射功能的光伏玻璃以避免这部分反射造成的入射能量损失，是很有意义的。目前的减反射技术各有优缺点，磁控溅射法特点是精确可控，但是成本很高，工艺上也限制了镀制多层的宽带减反射膜，虽然从科学研究的角度没有问题，但是产业化生产是不可行的；溶胶-凝胶法的优势是能够相对简单的实现大面积玻璃镀膜，但是其膜层的折射率较为均匀，不能得到宽带减反射的效果。通过对光学减反射理论、方法、玻璃减反射技术的调研分析，研究发现梯度折射率薄膜可以实现宽波段、大角度减反射等优良的特性，而腐蚀法是可以实现这种特性并且可以产业化的光伏玻璃减反射技术。
本文采用分层等效的方法，用一个由均质膜构成的多层膜堆等效梯度折射率减反射膜层，构造了多种折射率分布的梯度折射率减反射薄膜，使之能用现有的光学薄膜设计软件对其进行理论计算模拟研究。论文设计了不同的折射率曲线、层数、膜厚等参数，并将计算分为两个阶段：首先针对可见光波段，以110-880nm厚度、三种层数、五种类型折射率分布曲线为参数构建减反射薄膜进行计算，结果表明：在此计算相应参数条件下，200nm以上膜层厚度可以达到宽带减反射的效果；第二阶段，将研究扩展到整个太阳光谱，根据腐蚀法的实验特点和第一部分的结论，重新修订了300nm内的六个厚度、100-300范围内的五种层数、四种减反射曲线等参数。结果表明：在300nm厚度内100层以上层数变化的影响变得很小。在可见光波段和太阳光波段，余弦函数和直线函数分别具有最高的透过率分布，通过进一步计算可见光透射比和太阳光直接透射比这两个评价光伏效能的国标参数，余弦函数结果要好于直线或其它函数。
另外，根据德鲁德、洛仑兹-洛伦茨、麦克劳德三种理论对折射率和密度的关系推导，得出了物质密度和折射率的对应关系，据此，提出通过腐蚀法控制玻璃材料表面层的密度使之逐渐变化得到折射率逐渐变化的梯度折射率薄膜，模拟了该梯度减反射薄膜的减反射特性，并与实验结果进行了比较。
本文采用新的二次腐蚀梯度折射率薄膜制备方法，分两步对玻璃表层附近材料进行选择性脱除，第一步使用低浓度的酸液脱除表层的钙、钠等金属离子；第二步使用浓度较大的HF等酸液进一步脱除内层的金属离子，并且对光伏玻璃基体表层进行了由外而内的稀疏化处理，得到了一层厚度为200-300nm、具有渐变密度的多孔SiO2微结构层。这种微结构层的折射率呈梯度分布，并且具有优秀的减反射特性。在紫外到近红外1200nm的波长带宽内，透过率都在96%以上；在350-1084nm的双面总反射率小于1%(即单面反射率<0.5%)，其中624-922nm的双面反射率低于0.2%(单面反射率<0.1%)；并且390-1000nm波长带宽范围的平均透过率达到了99.22%。
本文测试了该光伏玻璃透过率随入射角度的变化，测试结果表明：梯度折射率减反射光伏玻璃在入射角为70°时的透过率达到了95.43%，比未做减反射的光伏玻璃透过率高出了约23%。本文对几种不同折射率梯度轮廓膜层的减反射光谱进行了模拟，并与实验测试结果对比，发现余弦函数分布的折射率与实验结果基本吻合。
通过对压花玻璃技术、溶胶-凝胶技术和腐蚀法技术三者的分析得知，压花玻璃与溶胶-凝胶技术的结合会填隙花纹从而削弱减反射效果，而腐蚀法制备技术不破坏压花玻璃表面微结构的减反射效果。本文在压花玻璃表面进行二次腐蚀法减反射的处理，得到了非常理想的减反射效果，在350至1100nm波长范围的透过率几乎达到了100%，是目前所有该类研究中最好的结果。膜层硬度测试表明，光伏玻璃减反射膜层的铅笔硬度达到2H以上，满足减反射光伏玻璃应用的基本要求。
Nowadays, the utilization of new energies with merits on clean, environmental-friendly,and efficient properties has met great opportunities and challenges under the dual difficultiesof solving global energy crisis and sustainable social development. Solar energy has been paidgrowing interests because it is a kind of unexhausted energy. The most efficient approachingof utilizing solar energy is photovoltaic conversion. In each solar cell, photovoltaic glass, aglass substrate with high optical transparency, is crutial for the enhancement oflight-to-electric conversion efficiency. In the traditional photovoltaic glass, at least8%ofincident light will be reflected by the polished surface (the reflectivity is27%at an incidentangle of70o). Therefore, it is of great significance to design a kind of photovoltaic glass withantireflection functions in order to avoid the incident light loss. Among current antireflectionapproaches, magnetron sputtering technique has a merit of controllability, but the fabricationexpense is high along with the limit in coating multilayer wide-band antireflection film.Althogh sol-gel method can be used to realize the preparation of antireflection films in a largerscale, the homogeneously distributed refractive index makes it unrealizable of wide-bandantiflection. After a careful survey in optical antiflection theories, methods, and techniques, weare inspired by the optical films with gradient refractive index (GRIN) to demonstrate thecharacteristics in wide-band, high-angle reflection et al. Among them, etching method is anefficient antireflection technique in realizing the above merits and commercialization.
In order to simulate the antireflection films with GRIN by the current design softwares onoptical films, we have successfully designed a kind of GRIN films by replacing ahomogeneous film by multilayer film piles, where there is a single refractive index in eachlayer. The simulation computation can be divided into two steps by adjusting refractiveindexes, number of layer, and film thickness:(i) For visible-light region, film thickness of110-880nm, three layers (35,70, and140), and five types of flectivity (linear, cosine,arccosine, and double quadratic functions) are employed to simulate the resultantantireflection films, indicating that the film thickness of higher than200nm is required toobtain the purpose of wide-band antireflection.(ii) Another films with six thicknesses, fivelayers ranging from100to300, and four antireflection curves are employed to investigate the antireflection properties in the full solar spectrum, suggesting a low effect beyond100layers.The results indicate that cosine function and linear function have maximum opticaltransparency in visible-light and infrared-light regions, respectively, whereas cosine functionis better than linear function in assessing the visible light transmittance ratio and solarspectrum direct transmission ratio.
Additionally, the relationships between density and refractivity are generated based onDrude, Lorentz-Lorenz, and Macleod theories. We launch an etching approach by controllingthe density of surface antireflection layer in order to obtain GRIN films and simulate theantiflection properties by sofewares.
The proposed etching route comprises two steps: in the first step, calcium and sodium etal metal ions are removed by low-concentrated aqueous solution; In the second step, thedeep-positioned metal ions are dissolved using high-concentrated aqueous solution to obtain alayer of porous SiO2structure with thickness of200-300nm after rarefaction treatments on thephotovoltaic glass substrate. The resultant SiO2structure exhibit gradiently distributedrefractive index and excellent antireflection properties, referring an optical transparency ofabove96%in the wavelength ranging from ultraviolet to near infrared (1200nm) regions. Thebifacial reflectivity is lower than1%in350-1084nm, and it is lower than0.2%in thewavelength range of624-922nm. Notably, an average optical transparency of99.22%isobtained in the wavelength ranging from390to1000nm.
The results on the dependence of optical transparency on incident angle indicate thetransparency is95.43%at an incident angle of70o, which is increased by23%in comparisonwith that for photovoltaic glass without antireflection. The antireflection spectra are finallysimulated by softwares and compared with experimental results, revealing a good agreement.
The combination of patterned photovoltaic glass with sol-gel technique can fill thepatterns and therefore decrease the antireflection functions, whereas etching method willremain good antireflection effect becaust it will not destroy the microstructures of patternedphotovoltaic glass. An optical transparency of nearly100%has been measured after dubleetching treatments. To the best of our knowledge, it is the highest record for photovoltaic glass.The hardness measurements reveal the resultant antireflection layer on photovoltaic glass has ahardness of higher2H, which can meet the requirement for the applications of antireflectionphotovoltaic glass.
引文
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